16/01/2011
Could you be tricked into believing a sugar pill will ease your pain? A brain scan could reveal whether you would respond to a placebo or not.

Tor Wager at the University of Colorado, Boulder, and colleagues took another look at two studies that involved scanning the brains of people given a painful stimulus. Each consisted of two trials where volunteers were given an ineffective cream to ease the pain. In one trial they were told it was a fake, in the other an analgesic.

When comparing brain responses from each trial, the group identified several brain structures that were more or less active before and during the painful stimulus in those who experienced a placebo effect. In placebo responders, activity dropped in areas processing pain, but increased in areas involved in emotion. This suggests that, rather than blocking pain signals into the brain, the placebo is changing the interpretation of pain.

The meaning of pain
In responders, "a lot of the action happens when people are expecting pain", Wager says. "What makes a placebo responder is the ability to re-evaluate the meaning of pain before it happens." His team created a map of relevant brain areas using information from 35 of the 47 participants. With this map they were better able to predict how much the placebo would diminish pain in the remaining participants.

The map could be useful for working out how much of a drug's effect is due to a placebo response in clinical trials and for identifying good candidates for placebo therapy. "It's difficult for experimental drugs to prove their superiority to placebo treatment and predicting placebo responders may help to deal with this challenge," says Luana Colloca at the National Institutes of Health in Bethesda, Maryland.

Journal of Neuroscience